So which is the smallest species of fish in the world?

02 Feb 2006 - Updated with comments from Maurice Kottelat and extracts from a news@nature.com post and the original paper. 31 Jan 2006 title of "A parasitic male anglerfish that's even smaller!" is now a subheading.

A parasitic male anglerfish that's even smaller!

Days after the flurry of reports about Paedocypris progenetica, world's smallest species of fish and vertebrate reached the world, a press release by the University of Washington (27 Jan 2006) brought attention to a 2005 paper that describes a 6.2mm sexually mature parasitic male anglerfish, Photocorynus spiniceps, attached to the back of a 46mm female. This claim to the world's smallest fish and vertebrate was listed in a review paper by T. W. Pietsch (see below).

The 6.2 mm long male Photocorynus spiniceps with the claim to the world's smallest vertebrate,was fused to the middle of the back of a 46 mm long female. Photo by T.W. Pietsch.

'Five of the 11 families of anglerfish exhibit sexual parasitism in which the much smaller males fuse for life with their mates by biting onto the sides, backs or bellies of a female. An attached male (2-8 in some species!) essentially turns the female into a hermaphrodite, providing her with the ability to reproduce while derives his nutrition from the female. The 6.2 mm male, for instance, has testes so huge they nearly fill his entire body cavity, crowding his other internal organs.'

The anglerfish paper was published in the September 2005 issue of Ichthyological Research which takes time to reach libraries. Meanwhile, the peat swamp fish paper, originally written in December 2004, was submitted, reviewed and accepted during that period.

Wait a minute, does smallest fish = smallest species?

Can the parasitic male alone claim record for an entire species? The females are much larger, and well above the list of contenders for smallest species. How does one define a small fish anyway? Maurice Kottelat had this to say:

"When we talk about small fish or vertebrate, we talk about species. E.g. In our paper, each time we use 'smallest' it is a relation with a species' name not with a specimen. [Read the discussion there on miniature fishes that begins, "A number of fish species discovered over the past few years have maximal known sizes ranging from 8.0 to 15.0 mm ... ].

Our paper discuss miniature fishes, for which there is a definition set by Weitzman & Vari (1988) Weitzman, S. H. & Vari, R. P. 1988. Miniaturization in South American freshwater fishes; an overview and discussion. Proc. Biol. Soc. Wash., 101: 444-465. The definition has to do with species which reproduce below 20 mm, or if reproduction data are not available, species less than 26 mm.

Since Photocorynus spiniceps does not reproduce below the 20 mm threshold, and unless it is demonstrated that the female is mature below that size, it is not a miniature species."

So who's the smallest fish in the world?

Meet Paedocypris progenetica, smallest fish and vertebrate organism in the world. Yes its time to reiterate the announcement from last week. The discussion was convincing enough and the claim holds. Paedocypris progenetica appears to regain its record-holding position.

Interestingly, when news@nature.com got a third party involved, he provided a alternative viewpoint (more appropriately described as a sucker punch); see: "Fish fight breaks out over tiny catch." By Michael Hopkin. news@Nature.com, 31 Jan 2006.

"Salamanders are the smallest vertebrates - there's not even any question," says David Wake of the University of California, Berkeley.

These amphibians are typically at least several centimetres long. But that is tiny in relation to the size of their genome, Wake points out. Of the 500 different salamander species, many have well over ten times as much genetic material in each cell as humans do. This makes for big, cumbersome cells, which means that adopting a complex body form is more of an achievement given the same body size, Wake argues.

"I'm always surprised that biologists who want to make sophisticated arguments resort to using a metre stick," he says. "It's not just millimetres that count - it's how you use those millimetres.""

Heh-heh.

Just as well the taxonomists aren't too heated up about the debate; after all there are a lot more fish in the ocean (and streams) and if habitats don't disappear, the enthusiastic work of taxonomists and systematists may reveal further claims to the title of the world's smallest fish.

"Even Ted Pietsch, the the University of Washington fish expert behind the harrumphing, says there's no sense in quibbling over the "smallest" title: "There are always difficulties in talking about the smallest - would that be length, volume or weight - the debate goes round and round." - "Big flap over smallest fish." By Alan Boyle. MSNBC Cosmic Log, 30 Jan 2006.

Records of the world's smallest fish and vertebrates

My curiosity was aroused - which other species of fish were earlier contenders for the title of the smallest vertebrate, where were they from and what ecosystems did they inhabit?

So I did a bit of googling. Deep water parasitic male angler fish, marine and brackish-water gobies and freshwater cyprinids - all smaller than shrimp and many insects! Unmaginable! Aquarists are busy joking about how these fish will choke their filters and more jokes about the fish that got away. Sigh.

For the record, the largest fish in the world is the plankton-eating Whale Shark (Rhincodon typus). Due to a demand for its fins, trade in its parts is regulated under the Convention on International Trade in Endangered Species (Cites). The species is listed as "vulnerable" in the IUCN Red List of threatened species.

2005 - 6.2mm Photocorynus spiniceps male with ripe testes. Specimen was collected in deep waters off The Philippines. Ref: Pietsch, T.W., 2005. Dimorphism, parastitism, and sex revisited: Modes of reproduction among deep-sea ceratioid anglerfishes (Teleostei: Lophiiformes). Ichthyological Research, 52(33): 207-236. Pdf of paper available for download at University of Washington Fish Collection. A1925 paper by Regan refers to a 7.3mm 'attached male' of the same species (listed in Pietsch's review) but I do not know if this was a sexually mature fish. If it was, it would have set the record that year to be eclipsed only in 2005. Ref: Regan CT (1925). Dwarfed males parasitic on the females in oceanic angler-fishes (Pediculati, Ceratioidea). Proc. R. Soc. Lond., B, 97:386-400.

Note that if you read the original papers, the size ranges provided for the entire material examined may include immatures. You have to look for statements about sexually mature individuals.

It was interesting to note that in many records, the point of actual collection, discovery and publication took between a few years to a couple of decades. A phenomenal diversity awaits the work of scientists, and this time lag highlights a fundamental role that museums serve in preserving, cataloguing, maintaining and holding expedition specimens until they can be described.

I am now wondering, will anything even smaller turn up next? Something even more incredible that we can hardly fathom? Or has it been wiped off the face of the earth already, and their habitats along with it?

"The whole exchange is quite amusing and in the end, what is really important is that we appreciate that there are still many areas on this globe that are unexplored, containing vast numbers of new species, among them very unusual ones that need to be discovered and their biology and anatomy explored."

"For some habitats we are facing a race against time, because they disappear faster than we can survey them."

One of the peat swamp habitats in Jambi, Sumatra,host to Paedocypris progenetica.Here today, gone tomorrow?